Color

• Author: Ronald K. Fierstein
• Profession: Lawyer on the team of litigators from the prestigious patent law firm of Fish & Neave
• Pages: 79-112

79

CHAPTER 5

COLOR

Leading Polaroid’s search to find a way to make a peel-apart color film

that could be used in existing Polaroid cameras, Howard Rogers had tried

to adapt the color coupler chemistry used by Kodak in all of its conven-

tional color films to the diffusion transfer process employed in Polaroid’s

one-step film. Consistent with his philosophy of “turning his people loose

[on scientific investigations] for long periods of time,”1 Land had allowed

Rogers to work without any interference for almost six years. Rogers,

having failed to make this adaptation work, embarked on a new path.

Color photographs are made by using combinations of three different

colored dyes familiar to anyone who buys printer cartridges today—cyan,

magenta, and yellow. In color film, each of these dyes is associated with

a photographic emulsion sensitive to the color opposite those dyes on the

visual spectrum—that is, red, green, and blue, respectively. The process

is known as subtractive color reproduction. Because white light is a com-

bination of all colors, it is necessary to subtract out unwanted colors to

reproduce a given color in a picture. For example, if the scene you are

photographing has a red cape, the photosensitive emulsion in the negative

sensitive to red light will react in the area in which the red cape is located.

This, in turn will cause the associated dye color—cyan, or blue—to be

subtracted from the image, leaving just magenta (red) and yellow in that

area that, when combined, will create the red color of the cape. A huge

range of subtle color combinations is required to reproduce what we see

in a color photograph.

In conventional color photography, color photographs are made by

first creating a negative that has all the opposite colors of the actual scene.

Color coupler imaging chemistry is used. In that system, the dyes are

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A Triumph of Genius

80

actually created during the development process—the couplers are chemi-

cal compounds capable of forming a colored dye by joining together with

molecules of developer that are oxidized in exposed regions of the film.

In chemistry terms, a molecule is “oxidized” when it loses an electron, an

event that primes it to react further. Only oxidized molecules of developer

can “couple”—non-oxidized molecules are inert. The developer mole-

cules are oxidized in exposed regions by reacting with the activated silver

halide. In the case of a red cape, in areas of the negative where red light

hits the photosensitive emulsion, the silver halide becomes activated. The

color developer, which is contained in the processing solution applied to

the exposed negative in the lab, becomes oxidized by the activated silver

halide in those regions. This allows the oxidized developer molecules to

“couple” with their associated color couplers to form cyan dye. Thus, in

the negative, the red areas of the picture look cyan.

By exposing that negative onto color photographic paper in a sub-

sequent operation, the colors are reversed back to the proper hues in the

final image, using the same imaging chemistry, which is this time incor-

porated into the photographic paper. In this instance, when white light is

passed through the negative, the red (magenta) component of the light

is absorbed by the cyan (blue) in the negative and so does not reach the

photographic paper. Only the green (yellow and blue combined) and blue

components of the light reach the paper, so that when the print is devel-

oped, magenta and yellow dye will be formed by the color couplers in that

positive area, resulting in a red image.

For Polaroid’s one-step process, a method had to be found to get the cor-

rect dyes to diffuse simultaneously from the negative to the image-receiving

layer in the film unit in order to produce a color photograph. After several

years of failed attempts, Rogers came up with a novel idea. He began inves-

tigating the use of a preformed dye attached to a molecule of developer that

could regulate its diffusion as a function of the latent image.2 He called these

new compounds “dye developers.” Rogers knew instinctively that he was

on the right track. “When an idea like this comes that you’re sure is good, it

spreads throughout your body,” Rogers explained years later. “I felt intoxi-

cated, but more ‘all there’ than usual—almost as if I were a giant. Then I

went to draw my new molecule for Land.”3

Rogers recorded the concept in his laboratory notebook in Septem-

ber 1953.4 He then worked for the next two years to find the right dye

developer compounds that could be employed to make a reliable diffusion

transfer film. Elkan Blout provided the necessary chemical expertise in

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Color 81

synthesizing those materials. In the process Rogers envisioned, the dye

developer molecule would initially be mobile in the processing solution,

but would become immobilized in exposed areas, leaving it unable to

transfer to the image-receiving layer. Thus, only dye developers in unex-

posed areas could transfer. During the same period, Rogers also realized

that a mirror image of this process was possible; that is, a dye developer

process could also be created in which the dye developer molecules were

initially immobile but would become mobilized when and where devel-

oped so they could transfer to form the image, usually by splitting off the

dye portion from the dye developer in exposed regions of the film.

Rogers named these molecules “negative” dye developers.5 The

other species, in which the dye developers were initially mobile and were

immobilized by development, were named “positive” dye developers. By

1955, Rogers had conducted successful image transfer experiments with

positive dye developers and had observed the effects of the process using

negative dye developers.6 He patented both.

Moving forward with his concept, Rogers focused his work on the pos-

itive dye developer version because it was the most practical one for his

immediate purposes. The emulsions needed to provide an image with cor-

rect colors using the positive dye developer process were the normal color

silver halide emulsions. But reproducing the correct colors in a scene using

negative dye developers required special photosensitive emulsions with

quite different color sensitivities. In the mid-1950s, these “reversal emul-

sions,” also known as “direct positive emulsions,” did not have sufficient

sensitivity for photographic usage and were employed only for document

copying purposes.7 Polaroid had received normal color emulsions from

Kodak to conduct its experiments. As was the case during the research on

the original one-step process in the 1940s, Rogers’ work was premised on

the assumption that such normal emulsions would be commercially avail-

able from Kodak at a reasonable cost when the time came to put together a

commercial product. Accordingly, Rogers needed to perfect the positive dye

developer process for a possible Polaroid one-step color film, an endeavor

that consumed him for the next two years. (See Fig. 5-1.)

The basic negative structure Rogers conceived was to put dye devel-

opers of the three image-forming colors in three separate layers behind

their three associated photosensitive silver halide layers in the film unit.

After exposure, the processing composition in the pod would be spread

within the film unit. The dye developer molecules would be soluble in

the processing composition and thus would be initially mobile, able to

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